Taken from the
2nd Volume of Joseph Priestley's"Experiments and Observations on
Different Kinds of Air"(This edition of Priestley's work was published in 3
volumes in 1790. I havetried to copy the original, page for page, as faithfully
as possible.)

B O O K IV.OF DEPHLOGISTICATED AIR.P A R T 1.

OF THE SOURCES OF DEPHLOGISTICATED AIR
SECTION 1.

An Account of the Discovery of dephlogisticated Air, and its general Properties.

THE contents of this section will furnish a very striking
illustration of the truth of a remark, which I have more than once made
in my philosophical writings, and which can hardly be too often repeated,
at it tends greatly to encourage philo- sophical investigations; viz. that
more is owing to what we call chance, that is, philosophically

p. 103

speaking, to the observation of events arising from
unknown causes, than to any proper design, or preconceived theory
in this business. This does not appear in the works of those who write
synthetically
upon these subjects; but would, I doubt not, appear very strikingly in
those who are the most celebrated for their philosophical acumen, did they
write analytically and ingenuously. For my own part, I will frankly acknowledge,
that, at the commencement of the experiments recited in this section, I
was so far from having formed any hypothesis that led to the discoveries
I made in pursuing them, that they would have appeared very improbable
to me had I been told of them; and when the decisive facts did at length
obtrude themselves upon my notice, it was very slowly, and with great hesitation,
that I yielded to the evidence of my senses. And yet, when I reconsider
the matter, and compare my last discoveries relating to the constitution
of the atmosphere with the first , I see the closest and the easiest connexion
between them, so as wonder that I should not have been led immediately
from the one to the other. That this was not the case, I attribute to the
force of prejudice, which unknown to ourselves, biases not only our judgements,
properly so called, but even the perceptions of our senses; for we may
take a maxim so strongly for granted, that

p. 104

the plainest evidence of sense will not entirely change,
and often hardly modify, our persuasions; and the more ingenious a man
is, the more effectually he is entangled in his errors; his ingenuity only
helping him to deceive himself, by evading the force of truth. There are, I believe, very few maxims
in philosophy that have laid firmer hold upon the mind, than that air,
meaning atmospherical air (free from various foreign matters, which were
always supposed to be dissolved, and intermixed with it) is a simple
elementary substance, indestructible, and unalterable, at least in
as much so as water is supposed to be. In the course of my inquiries, I
was, however, soon satisfied that atmospherical air is not an unalterable
thing; for that, according to my first hypothesis, the phlogiston with
which it becomes loaded from bodies burning in it and animals breathing
it, and various other chemical processes, so far alters and depraves it,
as to render it altogether unfit for inflammation, respiration, and other
purposes to which it is subservient; and I had discovered that agitation
in water, the process of vegetation, and probably other natural processes,
restore it to its original purity. But I own I had no idea of the possibility
of going any farther in this way, and thereby procuring air purer than
the best common air. I might, indeed, have naturally imagined that such
would be air that should contain less

p. 105

phlogifton than the air of the atmosphere; but 1 had
no idea that such a composition was possible. In my first publication on the subject
of air, I mentioned that which I had got from nitre, and the account
I then gave of it demonstrates it to have been dephlogisticated air, but
I had not pursued that experiment, nor was it of any use to me in the following
course. It may be worth while, however, to recite what I then observed,
which was as follows. All the kinds of factitious air, I
then observed, on which 1 had made the experiment, were highly noxious,
except that which is extracted from nitre, or alum; but in this even a
candle burned just as in common air. In one quantity which I got from nitre
a candle not only burned, but the flame was increased, and something was
heard like a hissing, similar to the decrepitation of nitre in an open
fire. The air was extracted from these substances
by heating them in a gun barrel, which was much corroded and soon spoiled
by the experiment. What effect this circumstance had upon the air I did
not consider. At the time of my first publication,
I was not possessed of a burning lens of any considerable force;
and for want of one, I could not possibly make many of the experiments
that I had

p. 106

projected, and which, in theory, appeared very promising.
I had, indeed, a mirror of force sufficient for my purpose. But
the nature of this instrument is such that it cannot be applied, with effect,
except upon substances that are capable of being suspended, or resting
on a very slender support. It cannot be directed at all upon any substance
in the form of powder, nor hardly upon any thing that requires to
be put into a vessel of quicksilver; which appears to me to be the most
accurate method of extracting air from a great variety of substances, as
was explained in the introduction to this work. But having afterwards procured
a lens of twelve inches diameter, and twenty inches focal distance, I proceeded
with great alacrity to examine, by the help of it, what kind of air a great
variety of substances, natural and factitious, would yield, putting them
into the vessels represented fig. a, Pl. IV. which I filled with quicksilver,
and kept inverted in a bason of the same. Mr. Warltire, a good chemist,
and lecturer in natural philosophy, happening to be at that time in Calne,
I explained my views to him, and was furnished by him with many substances,
which I could not otherwise have procured. With this apparatus, after a variety
of other experiments, an account of which will be found in its proper place,
on the 1st of August, 1774, I endeav- oured to extract air from mercurius
calcinatus perse; and I presently found

p.107

that, by means of this lens, air was expelled from it
very readily. Having got about three or four times as much as the bulk
of my materials, I admitted water to it, and found that it was not imbibed
by it. But what surprised me more than I can well express, was, that a
candle burned in this air with a remarkably vigorous flame, very much like
that enlarged flame with which a candle burns in nitrous air, exposed to
iron or liver of sulphur; but as I had got nothing like this remarkable
appearance from any kind of air besides this particular modific- ation
of nitrous air, and I knew no nitrous acid was used in the preparation
of mercurius calcinatus, I was utterly at a loss how to account
for it. In this case, also, though I did not
give sufficient attention to the circumstance at that time, the flame of
the candle, besides being larger, burned with more splendor and heat than
in that species of nitrous air; and a piece of red hot wood sparkled in
it, exactly like paper dipped in a solution of nitre, and it consumed very
fast; an experiment which I had never thought of trying with dephlogisticated
nitrous air. At the same time that I made the above-
mentioned experiment, I extracted a quantity of air, with the very same
property, from the common red precipitate, which being produced
by a solution of mercury in spirit of nitre, made me conclude that this
peculiar property,

p. 108

being similar to that of the modification of nitrous
air above mentioned, depended upon something being communicated to it by
the nitrous acid; and since the mercurius calcinatus is produced
by exposing mercury to a certain degree of heat, where common air has access
to it, I likewise concluded that this substance had collected something
of nitre, in that state of heat, from the atmosphere. This, however, appearing to me much
more extraordinary than it ought to have done, I entertained some suspicion
that the mercurius calcinatus, on which I had made my experiments,
being bought at a common apothecary's, might, in fact, be nothing more
than red precipitate; though, had I been any thing of a practical chymist,
I could not have entertained any such suspicion. However, mentioning this
suspicion to Mr. Warltire, he furnished me with some that he had kept for
a specimen of the preparation, and which, he told me, he could warrant
to be genuine. This being treated in the same manner as the former, only
by a longer continuance of heat, I extracted much more air from it than
from the other. This experiment might have satisfied
any moderate sceptic; but, however, being at Paris in the October following,
and knowing that there were several very eminent chymists in that place,
I did

p. 109

not omit the opportunity, by means of my friend Mr. Magellan,
to get an ounce of mercurius calcinatus prepared by Mr. Cadet, of
the genuineness of which there could not possibly be any suspicion; and
at the same time, I frequently mentioned my surprise at the kind of air
which I had got from this preparation to Mr. Lavoisier, Mr. le Roy, and
several other philosophers, who honoured me with their notice in that city;
and who, I dare say, cannot fail to recollect the circumstance. At the same time, I had no suspicion
that the air which I had got from the mercurius calcinatus was even
wholesome, so far was I from knowing what it was that I had really found;
taking it for granted, that it was nothing more than such kind of air as
I had brought nitrous air to be by the processes above mentioned; and in
this air I have observed that a candle would burn sometimes quite naturally,
and sometimes with a beautiful enlarged flame, and yet remain perfectly
noxious. At the same time that I had got the
air above mentioned from mercurius calcinatus and the red precipitate,
I had got the same kind from red lead or minium. In this
process, that part of the minium on which the focus of the lens had fallen,
turned yellow. One third of the air, in this experiment was readily absorbed
by water, but, in the remainder,

p. 110

a candle burned very strongly, and with a crackling noise. This experiment with red lead
confirmed me more in my suspicion, that the mercurius calcinatus
must: have got the property of yielding this kind of air from the atmosphere,
the process by which that preparation, and this of red lead is made, being
similar. As I never make the least secret of any thing that I observe I
mentioned this experiment also, as well as those with the mercurius
calcinatus, and the red precipitate, to all my philosophical acquaintance
at Paris, and elsewhere; having no idea at that time, to what these remarkable
facts would lead. Presently after my return from abroad,
I went to work upon the mercurius calcinatus, which I had procured
from Mr. Cadet; and, with a very moderate degree of heat, I got from some
of it, an ounce measure of air, which I observed to be not readily imbibed,
either by the substance itself from which it had been expelled (for I suffered
them to continue a long time together before I transferred the air to any
other place) or by water, in which I suffered this air to stand a considerable
time before I made any experiment upon it. In this air, as I had expected, a
candle burned with a vivid flame; but what I observed new at this time
(Nov. 19) and which surprised me no

p. 111

less than the fact I had discovered before, was, that,
whereas a few moments agitation in water will deprive the modified nitrous
air of its property of admitting a candle to burn in it; yet, after more
than ten times as much agitation as would be sufficient to produce this
alteration in the nitrous air, no sensible change was produced in this.
A candle still burned in it with a strong flame; and it did not, in the
least, diminish common air, which I had observed that nitrous air, in this
state, in some measure does. But I was much more surprised, when,
after two days, in which this air had continued in contact with water (by
which it was diminished about one twentieth of its bulk) I agitated it
violently in water about five minutes, and found that a candle still burned
in it as well as in common air. The same degree of agitation would have
made phlogisticated nitrous air fit for respiration indeed, but it would
certainly have extinguished a candle. These facts fully convinced me, that
there must be a very material difference between the constitution of the
air from mercurius calcinatus, and that of dephlogisticated nitrous
air, notwithstanding, their resemblance in some particulars. But though
I did not doubt that the air from mercurius calcinatus was fit for
respiration, after being agitated in water, as every kind of air without
exception, on which I had

p. 112

tried the experiment, had been, I still did not suspect
that it was respirable in the first instance; so far was I from having
any idea of this air being, what it really was, much superior, in this
respect, to the air of the atmosphere. In this ignorance of the real nature
of this kind of air, I continued from this time (November) to the 1st of
March following; having, in the mean time, been intent upon my experiments
on the vitriolic acid air, and the various modifications of air produced
by spirit of nitre. But in the course of this month, I not only ascertained
the nature of this kind of air, though very gradually, but was led by it,
as I then thought, to the complete discovery of the constitution of the
air we breathe. Till this first of March, I775, I
had so little suspicion of the air from mercurius calcinatus, &c.
being wholesome, that I had not even thought of applying to it the test
of nitrous air; but thinking (as my reader must imagine I frequently must
have done) on the candle burning in it after long agitation in water, it
occurred to me at last to make the experiment; and putting one measure
of nitrous air to two measures of this air, I found, not only that it was
diminished, but that it was diminished quite as much as common air, and
that the redness of the mixture was likewise equal to that of a similar
mixture of nitrous and common air.

p. 113

After this I had no doubt but that
the air from mercurius calcinatus was fit for respiration, and that
it had all the other properties of genuine common air. But I did not take
notice of what I might have observed, if I had not been so fully possessed
by the notion of there being no air better than common air, that the redness
was really deeper, and the diminution something greater than common air
would have admitted. I now concluded that all the constituent
parts of the air were equally, and in their proper proportion, imbibed
in the preparation of this substance and also in the process of making
red lead. For at the same time that I made the above mentioned experiment
on the air from mercurius calcinatus, I likewise observed that the
air which I had extracted from red lead, after the fixed air was washed
out of it, was of the same nature, being diminished by nitrous air like
common air; but, at the same time, I was puzzled to find that air from
the red precipitate was diminished in the same manner, though the process
for making this substance is quite different from that of making the two
others. But to this circumstance I happened not to give much attention. I wish my reader be not quite tired
with the frequent repetition of the word surprize, and others of
similar import; but I must go on in that style a

p. 114

little longer. For the next day I was more surprised
than ever I had been before, with finding, that, after the above mentioned
mixture of nitrous air and the air from mercurius calcinatus, had
stood all night (in which time the whole diminution must have taken place;
and, consequently, had it been common air, it must have been made perfectly
noxious, and intirely unfit for respiration or inflammation) a candle burned
in it, and even better than in common air. I cannot, at this distance of time,
recollect what it was that I had in view in making this experiment; but
I know I had no expectation of the real issue of it. Having acquired a
considerable degree of readiness in making experiments of this kind, a
very slight and evanescent motive would be sufficient to induce me to do
it. If, however, I had not happened, for some other purpose, to have had
a lighted candle before me, I should probably never have made the trial;
and the whole train of my future experiments relating to this kind of air
might have been prevented. Still, however, having no conception
of the real cause of this phenomenon, I considered it as something very
extraordinary; but as a property that was peculiar to air extracted from
these substances, and adventitious; and I always spoke of the air
to my acquaintance as being substantially the same

p. 115

thing with common air. I particularly remember my telling
Dr. Price, that I was myself perfectly satisfied of its being common air,
as it appeared to be so by the test of nitrous air; though, for the satisfaction
of others, I wanted a mouse to make the proof quite complete. On the 8th of this month I procured
a mouse, and put it into a glass vessel, containing two ounce measures
of the air from mercurius calcinatus. Had it been common air, a
full grown mouse, as this was, would have lived in it about a quarter of
an hour. In this air, however, my mouse lived a full half hour; and though
it was taken out seemingly dead, it appeared to have been only exceedingly
chilled; for, upon being held to the fire, it presently revived, and appeared
not to have received any harm from the experiment. By this I was confirmed in my conclusion,
that the air extracted from mercurius calcinatus, &c. was at
least as good as common air; but I did not certainly conclude that
it was any better; because, though one mouse would live only a quarter
of an hour in a given quantity of air, I knew it was not impossible but
that another mouse might have lived in it half an hour; so little accuracy
is there in this method of ascertaining the goodness of air: and indeed
I have never had recourse to it for my own satisfaction, since the discovery
of that most ready,

p. 116

accurate, and elegant test that nitrous air furnishes.
But in this case I had a view to publishing the most generally-satisfactory
account of my experiments that the nature of the thing would admit of. This experiment with the mouse, when
I had reflected upon it some time, gave me so much suspicion that the air
into which I had put it was better than common air, that I was induced,
the day after, to apply the test of nitrous air to a small part of that
very quantity of air which the mouse had breathed so long; so that, had
it been common air, I was satisfied it must have been very nearly, if not
altogether, as noxious as possible, so as not to be affected by nitrous
air; when, to my surprize again, I found that though it had been breathed
so long, it was still better than common air. For after mixing it with
nitrous air, in the usual proportion of two to one, it was diminished in
the proportion of four and a half to three and a half; that is, the nitrous
air had made it two ninths less than before, and this in a very short space
of time; whereas I had never found that, in the longest time, any common
air was reduced more than one fifth of its bulk by any pro portion of nitrous
air, nor more than one fourth by any phlogistic process whatever. Thinking
of this extraordinary fact upon my pillow, the next morning I put another
measure of nitrous air to the same mixture, and, to my utter astonishment,
found that

p. 117

it was farther diminished to almost one half of its original
quantity. I then put a third measure to it; but this did not diminish it
any farther; but, however, left it one measure less than it was even after
the mouse had been taken out of it. Being now fully satisfied that this
air, even after the mouse had breathed it half an hour, was much better
than common air; and having a quantity of it still left, sufficient for
the experiment, viz. an ounce measure and a half, I put the mouse into
it; when I observed that it seemed to feel no shock upon being put into
it, evident signs of which would have been visible, if the air had not
been very wholesome; but that it remained perfectly at its ease another
full half hour, when I took it out quite lively and vigorous. Measuring
the air the next day, I found it to be reduced from one and a half to two
thirds of an ounce measure. And after this, if I remember well (for in
my register of the day I only find it noted, that it was considerably
diminished by nitrous air) it was nearly as good as common air. It
was evident, indeed, from the mouse having been taken out quite vigorous,
that the air could not have been rendered very noxious. For my farther satisfaction I procured
another mouse, and putting it into less than two ounce mea- sures of air
extracted from mercurius calcinatus and

p. 118

air from red precipitate (which, having found them to
be of the same quality, I had mixed together) it lived three quarters of
an hour. But not having had the precaution to set the vessel in a warm
place, I suspect that the mouse died of cold. However, as it had lived
three times as long as it could probably have lived in the same quantity
of common air, and I did not expect much accuracy from this kind of test,
I did not think it necessary to make any more experiments with mice. Being now fully satisfied of the superior
goodness of this kind of air, I proceeded to measure that degree of purity,
with as much accuracy as I could by the test of nitrous air; and I began
with putting one measure of nitrous air to two measures of this air; as
if I had been examining common air; and now I observed that the diminution
was evidently greater than common air would have suffered by the same treatment.
A second measure of nitrous air reduced it to two thirds of its original
quantity, and a third measure to one half. Suspecting that the diminution
could not proceed much farther, I then added only half a measure of nitrous
air. By this it was diminished still more} but not much, and another half
measure made it more than half of its original quantity; so that, in this
case, two measures of this air took more than two measures of nitrous

p. 119

air, and yet remained less than half of what it was.
Five measures brought it pretty exactly to its original dimensions. At the same time, air from the red
precipitate was diminished in the same proportion as that from mercurius
calcinatus, five measures of nitrous air being received by two measures
of this without any increase of dimensions. Now as common air takes about
one half of its bulk of nitrous air, before it begins to receive any addition
to its dimensions from more nitrous air, and this air took more than four
half measures before it ceased to be diminished by more nitrous air, and
even five half measures made no addition to its original dimensions, I
concluded that it was between four and five times as good a common air.
It will be seen that I have since procured air purer than this.